1. Field of the Invention
The present invention relates to high thermal conductivity, nonbleeding greaselike compounds having a high packing density which are used primarily as a heat transfer means for cooling electronic components such as VLSI chips.
2. Description of the Prior Art
With the high density capabilities and fast switching speeds afforded by VLSI chips, various improved means of dissipating the unprecedented amounts of heat generated by VLSI chips and other solid state components have been pursued. One such means is a thermal compound commonly referred to as thermal grease, which is most commonly used to conduct heat from area arrays of solder-ball attached integrated circuit chips of a packaging module to a heat conducting means or a heat sink as shown in FIG. 1. As VLSI density increases, so does the power of each chip and the associated packaging module when multichip modules are used. The requirements associated with cooling high-powered chips (e.g., state of the art bipolar chips may generate 60 watts per square centimeter area or greater) dictate that the thermal conduction compounds such as thermal grease must have a high thermal conductivity (and preferably being an electrical insulator) while being of low viscosity and compliant so that the thermal compound can be easily applied to the surfaces of the chips to be cooled and can conform ultimately with the microscopically rough surface of the chips, which are often bowed or tilted to minimize air gaps which are detrimental to the cooling process. Low viscosity is also required because the chips and solder bonds that usually attach the chips to a substrate are fragile and the force applied by the thermal grease to the chip must be minimal in magnitude while maximum in contact to minimize interface thermal resistance so that a good thermal path is formed. It is a further requirement that the thermal grease compound be able to withstand power cycling at high chip powers with the attendant mechanical stresses arising from the differences in coefficients of thermal expansion of the various material systems over the life of a module without the compound degrading significantly in thermal conductivity or mechanically, such as experiencing phase separation between the liquid and solid components of the compound.
There are numerous thermal grease compounds available in the art. Heretofore, however, none have satisfied the combined requirements of high thermal conductivity, high electrical resistivity, low viscosity, compliance, and resistance to phase separation or degradation in property stability (thermal conductivity, viscosity, etc.) of the present invention. For example, U.S. Pat. No. 3,405,066 teaches the use of particles such as boron nitride or silicon dioxide in a dielectric fluid such as mineral oil for the purposes of conducting heat from electrical devices and equipment. The use of the '066 thermal grease in state of the art VLSI systems wherein chip powers exceed 30-60 watts per square centimeter has been found not to be adequate because the heat conductive particles separate from the dielectric liquid carrier when the chips experience fluctuative power cycling during this operation, (chips experience temperature differences of greater than 50 degrees centigrade between the inactive and fully active states). This phase separation leads to a decrease in thermal conductivity wherein the thermal compound would eventually not be adequate to dissipate the required amount of heat from the semiconductor chips.
U.S. Pat. No. 3,882,033 to Wright teaches that organopolysiloxane grease compositions have good dielectric and heat transfer properties can be obtained by utilizing certain proportions of polysiloxane fluid, a dielectric desiccant selected from anhydrous calcium sulfate and synthetic zeolites, and a grease thickening and thermal conducting agent. Materials such as anhydrous calcium sulfate and synthetic zeolite have lower intrinsic thermal conductivities compared with the particles described in the present invention. Also, no means or process is shown for achieving high particle packing density to further increase thermal conductivity or to inhibit separation of the particles from the liquid carrier in such pastes in contact with chips operating at high and fluctuating power cycling.
A flexible heat conducting sheet having thermally conducting boron nitride particles dispersed within is described in the IBM TDB dated Apr. 1983, pp. 5740-5743 by Lacombe et al. Lacombe et al. used polyisobutylene (PIB) as the organic carrier which has a very high loading density of boron nitride particles in the carrier. This is not suited for the present application wherein low viscosity and compliance is required in a mobile dielectric medium so that the thermal compound can intimately conform to the semiconductor devices to be cooled.
The thermal compound must also be applied as a thin layer so that the thermal path is as small as possible. The small gap results in excessive mechanical shear stress on the thermal compound that, when combined with thermal stress from high temperatures and power cycling, causes phase separation during power cycling of thermal compounds heretofore known. Low viscosity is also required to accommodate chip tilt and any chip surface irregularities while the thermal grease compound must exhibit rapid stress relaxation to limit the amount of force transmitted to the chips.
IBM TDB dated Mar. 1983, pp. 5322 by Mondou et al describes the use of boron nitride particles in a poly(alpha-olefin) carrier with wetting agents incorporated therein. The particles in Mondou et al are not at a higher surface energy than the carrier, thus wetting of the particle surfaces by the organic carrier is not spontaneous. This reference does not suggest the unique characteristics required and taught by the present invention, which allow for high thermal conductivity, high electrical resistivity, low viscosity, and chemical stability (does not oxidize or cause corrosion, keeps thermal conductivity and viscosity relatively constant) while eliminating phase separation during power cycling. A similar thermal grease is described by Mondou et al in the IBM TDB dated Mar. 1983, pages 5320-21 wherein it is indicated that the thermal conductivity is greater than 1.25 Watts per meter-degree C (W/m-.degree. C.). This compound also will not provide high thermal conductivity while also providing low viscosity and eliminating phase separation during power cycling of the high powered chips required in state of the art VLSI and VLSI applications.
IBM TDB to Aakalu et al. dated Dec. 1981, pp. 3530 employs a thermally conductive powder dispersed in a mobile hydrocarbon fluid, resulting in a dielectric medium. Aakalu et al. teach the use of hydrated silica to enhance the resistance of the thermal grease to phase separation. The thermal conductivities achieved by this thermal grease are in the range of 1 Watt per meter-degree C with 71.4 weight percent boron nitride loading. This relatively high loading results in a paste having a viscosity that causes the paste to be not mobile enough to be placed into thin gaps without threatening either cracking large area chips and/or their associated solder bonds when applied thereto. In addition to the relative high viscosity of the Aakalu et al. TDB, it has been found by the inventors that phase separation occurs if this type of compound is powered at high fluctuating power cycling levels, causing a mechanical shearing of the applied thin film of the thermal grease compound. i.e. in the range of greater than 30 to 60 W/cm.sup.2.
U.S. Pat. No. 4,265,775 to Aakalu et al. describes a thermal filler powder of laminar or dendridic shapes in a silicone liquid carrier which incorporates silica fibers to help prevent bleeding of the particles from the liquid carrier due to its high surface area. Even though this disclosure inhibits bleeding for certain applications, it has been found that at repeated power cycling and chip temperatures over 80.degree. C. that bleeding is not prevented by the addition of mere silica alone. Moreover, the wetting agents and liquid carrier described in the '775 thermal compound are not suitable for the present application because they cannot be removed completely by solvents; and thus causes metallurgical non-wetting problems during rework of solder joints, and contamination of other surfaces in multichip packages and the tooling (i.e. furnaces, etc.) used for assembling such packages.
In view of the above there exists a need in the art for stable thermal conducting compounds having a high thermal conductivity and high electrical resistivity, while also having a relatively low viscosity so that compound exists as a mobile medium which can easily be applied and conformed to, and wet the surface of the chips to be cooled while not exerting forces to crack the chips or solder bonds which attach the chips to substrates. It is also required that there be no phase separation between the liquid carrier and thermally conductive particles, or degradation in viscosity or thermal conductivity during power cycling of high powered VLSI and VLSI chips and that the thermal compounds are capable of being applied in thin layers so that the total thermal resistance path through the thermal compound is as low as possible. The thermal compound must also be capable of withstanding reciprocating mechanical shear stress during power cycling. Such thermal mechanical stressing occurs when the thermal compound is in the small gap between a chip and an internal thermal enhancement such as a spring loaded piston as shown in FIG. 1B, or between the chip and a cap as shown in FIG. 1A. There is also a need for the compound to be readily cleanable from chips and metal surfaces to facilitate rework of chips.